1. Field of the Invention
The present invention relates to novel catalysts, and, more especially, to novel catalysts comprising a support substrate having an oxide layer deposited thereon, said oxide layer being more firmly and permanently adhered to its support by means of a gum or biogum.
2. Description of the Prior Art
A wide variety of catalysts are known to this art consisting of a substrate coated with a film or layer of refractory oxides, onto which a catalytically active phase is deposited. These are useful in numerous gaseous phase reactions. For example, the catalytic conversion of internal combustion engine exhaust gases, whether gasoline or diesel, is presently typically accomplished using catalysts comprising a ceramic or metallic substrate in the form of an inert and rigid (monolithic) structure having a honeycomb configuration coated with a film or layer of oxides, such as alumina, onto which the active phase is deposited.
Monolithic supports are generally produced by depositing one or more layers of oxides onto the monolithic substrate. Such a process is described, for example, in French Patent No. 2,512,004, assigned to the assignee hereof.
In the process described in this patent, an aqueous suspension is formed from a mixture of alumina powders. This mixture includes a charge and a binder, the latter consisting essentially of the dispersible fraction of the mixture. The substrate is immersed in the aqueous suspension, then permitted to drip and drain, and is next dried and then calcined.
The mechanical properties and the adherence of the layers deposited in this manner are an important factor in the quality of the catalyst. It is known to this art that the monolithic catalysts that are particularly useful for vehicular catalytic converters are subjected to high temperatures, great variations in temperature, and considerable mechanical vibrations while in use.
These thermal and mechanical stresses tend to cause peeling of the catalyst layers and thus a loss in the amount of oxides deposited, which obviously gives rise to a reduction in the effectiveness of the catalyst over the long term.